Fibre reinforced composites



Nov. 24, 1970 J. c. CANNELL ET AL 3,541,659

FIBRE REINFORCED COMPOSITES Filed March 18, 1968 O o N Lu 7 (I E m l O. E E 1 0 l -Q I I I 1 I I l 5v c Q m N 'NI'DS/SNOJ. 'Sl'fl I vent .s' ou/v Com/e46 Gem Wei; Woe. a ly/Wes ihwg/vrr a wa mmd Attorneys United States Patent Oflice 3,541,659 Patented Nov. 24, 1970 3,541,659 FIBRE REINFORCED COMPOSITES John Corjeag Cannell, Waltham Abbey, and Noel James Parratt, Loughton, England, assignors to Minister of Technology in Her Britannic Majestys Government of the United Kingdom of Great Britain and Northern Ireland, London, England Filed Mar. 18, 1968, Ser. No. 713,778 Claims priority, application Great Britain, Mar. 16, 1967, 12,465/ 67 Int. Cl. C22c 29/00 US. Cl. 29-182.5 Claims ABSTRACT OF THE DISCLOSURE A reinforced metal composite has a matrix containing fibres, the matrix comprising aluminium powder containing between about 5% and by weight of aluminum oxide dispersed within individual particles.

The invention relates to fibre reinforced composites and their manufacture.

It is known that the mechanical, particular the tensile, properties of various plastics and metals can be greatly enhanced by reinforcing them with filaments of a strong and/ or stiffer material, often with a reduction in density. The filaments should have an average length suflicient for adequate transfer of stress to be obtainable between the two materials. Suggested filamentary materials include metal wires, ceramic, mineral or carbon fibres or whiskers.

The realization of useful reinforced metal matrices is hampered by several problems. The matrix and reinforcing materials must be chemically and physically compatible in the range of temperatures within which the composite is to be employed. There are at least two possible conditions of instability which may render the materials incompatible chemically. The one material may react chemically with the other in a detrimental manner, or the reinforcing material may diffuse through the matrix causing loss of filamentary form. The two materials may be physically compatible when, for example, the composite cannot be fabricated because the filaments cannot withstand the temperature or mechanical stress imposed, or the two materials so differ in thermal expansion coefficient that a useful composite cannot be made.

Composites have been made of whiskers or like filamentary material in a ductile metal matrix using a powder of the metal. These composites have proved anelastic, and so rapidly loss their tensile strength with use at elevated temperatures that they often display little advantage over alloys whose base metal is the same as that of the powder.

It has now been discovered that, contrary to widely held opinion, metal matrices almost totally lacking ductility may be used as a matrix in a satisfactory composite. The present invention provides such a composite having high strength and adequate toughness, and which retains these properties to a marked degree at elevated temperatures.

According to the present invention, therefore, there is provided a reinforced metal composite in the form of a matrix containing fibres, the matrix comprising a aluminum powder containing between about 5% and 25% by weight of aluminum oxide dispersed within the individual particles.

For volume fractions of fine fibres (up to several microns diameter) about 10% suitable aluminium powder is available commercially in the form of flakes with the aluminium oxide dispersed as layers substantially within each flake. Atomised aluminium would be suitable provided the particulars were of submicron size. For fine fibre volume fractions less than 10% a more suitable matrix material is the partly agglomerated commercial product known as sintered aluminum powder (S.A.P.).

Typical fibres include alumina fibres, silicon nitride and silicon carbide whiskers, and carbon fibres, all of which have elastic moduli of the order of 60 million lb./ sq. in.

Whereas composites of these fibres in simple aluminium matrices demonstrate recurring permanent set on repeated loading at very small strains, the presence of the oxide phase in the composite with aluminium powder greatly reduces this effect.

The elastic limit of the compostie is continuously increased with oxide content, but the matrix begins to become too brittle if the oxide content dispersed Within the individual particle is above about 25 The preferred range of oxide content is 6-15 The oxide may be dispersed as particles or layers within each particle of the powder, although for the most part, when aluminium is supplied commercially the oxide is dispersed as layers within each flake, the separation of each layer being between and 3,000 angstrom units.

The size of the aluminium particles (which term includes the particle agglomerates of S.A.P.) has a great effect upon the ease with which the composite can be fabricated and upon the stress transfer obtainable between the fibres and the matrix. Sufiicient particles should be available of a size comparable With the diameter of the reinforcing fibres in order to adequately encapsulate the fibres. Thus, generally coarser powders such as S.A.P. are more tolerable with fibres such as carbon having a diameter of about 8 microns (,u) than with silicon nitride whisker fibres having a diameter of 12,u.

For aluminium flakes the preferred dimensions generally are less than 1/2,LL thickness and less than 4 breadth whereas for SAP. the particle size distribution is satisfactory if 20% by weight are less than 10 diameter and 10% by weight are less than 21/. diameter.

In accordance with the invention a reinforced metal composite is produced by subjecting a mixture of reinforcing fibres and aluminium powder containing a dispersion of about 5% to 25% of aluminium oxide to high temperature and pressure, the temperature being greater than about 500 C. and less than that at which would occur either a deleterious reaction between or breakdown of the materials. The processes may be performed by extruding the mixture with particular advantage since alignment of the reinforcing fibres is then obtained. In general S.A.P. is to be preferred in an extrusion process whereas aluminium flakes are more useful in other composite forming processes.

Generally the properties of the composite tend towards those of the fibres themselves with increase of their proportion in the composite with the possible exception of brittleness. Increase in the proportion of fibres also raises the pressure required to form the composite and the extent of damage to the fibres, which decreases their effect in the composite.

The tensile properties of a particular composite in accordance with the invention will now be discussed with reference to the accompanying drawing which is a graph of the relationship between strength and temperature of the composite compared with known materials of similar density.

A composite of about 20% by Weight silicon nitride (Si N whiskers well mixed in aluminium flake containing nominally 6-15 aluminium oxide was made by heating the mixture to about 500 C., applying slight pressure i.e. Mi-V: tons/sq. ins., increasing both temperature and pressure simultaneously to about 625 C. and 34 tainable with a commercially extruded sintered aluminium powder are indicated by the curve B while those for a flake aluminium powder alone consolidated in the above described manner, as shown by curve C. The curves B and C show tensile strengths lower than those of curve A by about 4 tons/ sq. in. and 10 tons/ sq. in. respectively at similar temperatures.

The curve D shows the behaviour of an aluminum alloy containing 2-2.5% copper, 05-15% nickel, 1.2-1.8% magnesium, 1-1.5% iron, O-0.3% silicon and 0.2% titanium. This alloy is known as Hiduminium-RR.5 8 alloy. It has a tensile strength of about 29 tons/sq. in. at ambient temperatures, and this is substantially maintained up to about 150 C. although the strength at this temperature is already lower than that of the composite. At about 180 C., however curve D depicts a rapid loss of strength and the alloy has a U.T.S. of about 7 tons/sq. in. at 300 C.

The graph shows clearly that while the Hiduminium RR.58 alloy which is specified for use in aircraft up to 180 C., retains a U.T.S. of over 16 tons/sq. in. up to about 200 C., the fibre reinforced aluminium/ aluminium oxide composite is capable of maintaining a U.T.S. of over 16 tons/sq. in. up to 300 C.

A further comparison between an extrusion of unreinforced sintered aluminium powder (S.A.P. 865 containing 13 /2 by weight oxide) and a composite prepared in accordance with the invention from SAP 865 and 5 volume percent of silicon nitride whiskers gave the following results (Tensile strengths measured in tons/sq.

4 Fatigue life measurements are the number of cycles to failure under 9 tons/sq. in. rotating bend cantilever.

We claim: 1. A reinforced metal composite in the form of a matrix containing fibres, selected from the group consisting of silicon nitride, silicon carbide, alumina, and carbon fibres, said matrix aluminium powder containing between about 5% and 25% by weight of aluminium oxide dispersed within individual particles of said aluminium powder.

2. A reinforced metal composite according to claim 1 wherein the proportion of aluminium oxide is 6-15 3. A reinforced metal composite according to claim 1 in which said aluminium powder is selected from the group consisting of aluminium flake in which the flake dimensions are less than /2 a thickness and less than 4 breadth; sintered aluminium powder in which at least 20% by weight of the particles are less than 10 diameter and at least 10% by weight are less 2 1 diameter; and atomised aluminium in which the particles are of submicron size.

4. A reinforced metal composite according to claim 1 wherein said fibres comprise up to about 20% by Weight of the composite.

5. A reinforced metal comprising according to claim 1 wherein said fibres have elastic moduli of the order of million pounds per square inch.

References Cited UNITED STATES PATENTS 3,114,197 12/1963 Du Bois et al. 29-1822 3,191,734 6/1965 Batchelor et al. 19268 3,297,415 1/1967 Allen 29191.6 3,364,975 1/1968 Gruber 16455 3,421,862 1/ 1969 Shyne 205 BENJAMIN R. PADGETI, Primary Examiner A. J. STEINER, Assistant Examiner U.S. Cl. X.R. 

